Vehicle drive axles typically include a pair of axle shafts for driving vehicle wheels. The drive axle uses a differential assembly to control input speed and torque to the axle shafts. Under ideal conditions, when the vehicle is driven along a straight path, the wheels will be turning at approximately the same speed and the torque will be equally split between both wheels. When the vehicle negotiates a turn, the outer wheel must travel over a greater distance than the inner wheel.
The differential assembly allows the inner wheel to turn at a slower speed than the outer wheel as the vehicle turns. Power is transmitted from a vehicle drive shaft to a drive pinion that is in constant mesh with a ring gear. The ring gear is bolted to a differential case that turns with the ring gear. A differential spider having four (4) support shafts orientated in the shape of a cross, has four (4) differential pinions. One differential pinion is supported for rotation on each support shaft. Power is transferred from the differential case to side gears that are splined to the axle shafts. The side gears are in constant mesh with the differential pinions. The outer ends of the axle shafts are bolted to the wheel hubs to which the wheels are mounted.
When the vehicle is driven in a straight path the ring gear, differential case, spider, differential pinions, and side gears all rotate as one unit to transfer power to the axle shafts. There is no relative movement between the differential pinions and the side gears. When the vehicle executes a turning maneuver, the differential pinion gears rotate on their respective shafts to speed up the rotation of one axle shaft while slowing the rotation of the other axle shaft.
Often the differential assembly includes a differential locking or biasing mechanism. When there are poor traction conditions, e.g., slippery or rough surfaced roads, the locking mechanism allows maximum wheel traction for improved control. If the differential assembly does not have the locking mechanism and one tire is on ice, the available traction torque on the opposite wheel is same as on the wheel on ice. Thus, the tire just spins on the ice and the vehicle is prohibited from traveling forward. The locking mechanism allows the axle shafts to rotate at the same speed while transferring most of the available torque to the tire not on the ice. If the tractive effort at this tire is sufficient, the vehicle can be moved off of the ice. When the mechanism is activated, power is transmitted through the differential gearing, and locking mechanism rather than through the differential gearing only.
One type of the locking mechanism includes a wet disc clutch that locks the differential case to the axle shafts, until a predetermined torque level is exceeded. The wet disc clutch includes a plurality of stationary discs interspersed with rotating discs in a fluid chamber. A piston applies a force to the wet disc clutch to compress the rotating and stationary discs of the wet disc clutch together to apply torque between the differential case to be locked to the axle shafts. The terms stationary and rotating applied to the disc are relative to the differential case.
One disadvantage with a typical wet disc clutch system is fluid leakage. The leakage problem results from the pressurized fluid transfer from stationary members to rotating members to actuate the piston. Complicated rotating seal units, sometimes comprising leak-by recapture circuits, must be incorporated into the differential assembly, which take up valuable packaging space and are expensive. The recapture system recovers the leaked fluid and returns it to a pump that is used for applying pressure to actuate the wet disc clutch. Another disadvantage is that the clutch torque capacity is limited by the discs and actuator assembly that can be physically fit within the differential case.
Thus, it is desirable to have a compact actuator assembly for a differential locking mechanism that can deliver significant pressure from a stationary source to a rotating source while eliminating leakage and overcoming other deficiencies in the prior art as outlined above.